The present invention generally relates to CMOS active pixel photodetectors and more particularly to an improved CMOS active pixel photodetector array which includes scavenging diodes between adjacent photodetectors, which prevents cross-talk between adjacent photodetectors, and prevents diffusion of thermally generated electrons from isolation regions into the photodetectors.
To overcome the limitations of CCD-based imaging circuits, more recent imaging circuits use complementary metal oxide semiconductors (CMOS) active pixel sensor (APS) cells to convert a pixel of light energy into an electrical signal. With active pixel sensor cells, a conventional photodiode is typically combined with a number of active transistors which, in addition to forming an electrical signal, provide amplification, readout control, and reset control.
FIG. 1 (from U.S. Pat. No. 5,970,316, incorporated herein by reference) is a schematic diagram that illustrates a conventional CMOS active pixel sensor cell 10. As shown in FIG. 1, the cell 10 includes a photodiode 12, a reset transistor 14 whose source is connected to photodiode 12, a source-follower transistor 16 whose gate is connected to photodiode 12, and a row-select transistor 18 whose drain is connected in series to the source of source-follower transistor 16.
Operation of active pixel sensor cell 10 is performed in three steps: a reset step, where cell 10 is reset from the previous integration cycle; an image integration step, where the light energy is collected and converted into an electrical signal; and a signal readout step, where the signal is read out.
As shown in FIG. 1, during the reset step, the gate of reset transistor 14 is briefly pulsed with a reset voltage VR (5 volts). The reset voltage VR turns on reset transistor 14 which pulls up the voltage on photodiode 12 and the gate of source-follower transistor 16 to an initial reset voltage. The initial reset voltage placed on the gate of source-follower transistor 16 also defines an initial intermediate voltage on the source of source-follower transistor 16 which is one threshold voltage drop less than the initial transfer voltage. Immediately after the gate of reset transistor 14 has been pulsed, the gate of row-select transistor 18 is pulsed with a row-select voltage VRS. The row-select voltage VRS on the gate of row-select transistor 18 causes the initial intermediate voltage on the source of source-follower transistor 16 to appear on the source of row-select transistor 18 as an initial integration voltage which, in turn, is read out and stored by an imaging system.
During integration, light energy, in the form of photons, strikes photodiode 12, thereby creating a number of electron-hole pairs. Photodiode 12 is designed to limit recombination between the newly formed electron-hole pairs. As a result, the photo-generated holes are attracted to the ground terminal of photodiode 12, while the photo-generated electrons are attracted to the positive terminal of photodiode 12 where each additional electron reduces the voltage on photodiode 12. Thus, at the end of the integration step, the potential on photodiode 12 and the gate of source-follower transistor 16 will have been reduced to a final integration voltage where the amount of the reduction represents the intensity of the received light energy. As above, the final integration voltage on the gate of source-follower transistor 16 defines a final intermediate voltage on the source of source-follower transistor 16.
Following the image integration period, the final intermediate voltage on the source of source-follower transistor 16 is then read out as a final integration voltage by again pulsing the gate of row-select transistor 18 with the row-select voltage VRS. As a result, a collected voltage which represents the total charge collected by cell 10 can be determined by subtracting the final integration voltage from the initial reset voltage.
FIG. 2 is a schematic top view of an active pixel 10. The cell 10, is preferably formed upon a p-type substrate. The photodiode 12 preferably comprises a lightly doped n-type region within the p-type substrate. In FIG. 2, the gate of the row select transistor 18 is shown as item 23. Similarly, the gate of the reset transistor 14 is shown as the reset gate 26. The gate of the source-follower transistor 16 is shown as item 20. The source (voltage supply, VDD) for the cell is shown as item 21.
FIGS. 3 and 4 illustrate an array of such cells 10. More specifically, FIG. 4 is a top view similar to FIG. 2 and FIG. 3 is a cross-sectional view along line A-Axe2x80x2. Such an array suffers from the disadvantage of color cross-talk. More specifically, as shown in FIG. 3, photoelectrons 30 produced by photons which naturally land between the cells 10 in the isolation region 31 have a tendency to migrate toward adjacent photodetectors 12. In the case of a color image sensor, where a color filter array pattern is used in conjunction with the pixel array, photons which have passed through a first pixel with a first color can impinge on the isolation region in between the first and second pixels and produce photoelectrons which end up in the photodetector of the second adjacent pixel. Since the signal integrated in the second adjacent pixel is ideally desired to represent the amount of photons passing through a second color filter above that pixel, any photoelectrons that reach the second photodetector that were created by photons passing through the first color filter cause color mixing or color cross-talk. Accurate representation of color image requires minimal color mixing. If substantial color mixing or cross-talk occurs, this will produce a hue shift in the resulting processed color image, and degrade color fidelity.
Color cross-talk or color mixing can also occur if photoelectrons bloom from the first pixel and flow over the isolation region into the second adjacent pixel. This can occur if the incident light of the first pixel is bright enough to completely fill the first photodetector, so that excess photoelectrons are created. These excess photoelectrons can then spill or bloom from the first pixel, traverse the isolation region 31 and be collected in the photodetector of the second adjacent pixel or pixels. This produces photoelectrons generated by photons associated with a first color being collected in a pixel or photodetector associated with a second color.
The direction of this cross-talk or color mixing will depend on the exact physical structure and layout of the pixel. In the case of the pixel array shown in FIG. 4, the mixing or cross-talk in the vertical direction is low due to the large space between photodiodes, and the effective drain of photoelectrons through the n+-substrate diode of the voltage supply 21. To the contrary, cross-talk in the horizontal direction is high due to the lack of a light shield to keep photoelectrons away from the pixel boundaries and the small isolation area/space 31 between adjacent photodiodes.
Isolation regions 31 are also typically areas that have a high thermal generation rate of electrons, typically referred to as dark current. These thermally generated electrons can diffuse or migrate from the isolation regions into adjacent photodetectors. These dark current electrons are then mixed with and read out with the photo-generated electrons. Since the thermally generated electrons are not associated with the incident optical signal, they are referred to as noise electrons, and degrade the signal to noise ratio performance of the pixel and image sensor.
Therefore, there is a need for a structure which removes photo-generated or thermally generated electrons which are created in the isolation area 31, before they can migrate to the photodetectors 12.
It is, therefore, an object of the present invention to provide a camera/imaging system having a complementary metal oxide semiconductor active pixel image sensor, comprised of an array of pixels, where each of the pixels includes a photodetector, a voltage source for supplying voltage to the photodetector, and a scavenging diode adjacent the photodetector. The scavenging diode can be connected to the voltage source of the circuit or a different voltage source. The scavenging diodes are positioned between adjacent photodetectors of the active pixel image sensor and prevent electrons generated within the isolation regions outside of the photodetector from migrating to the photodetector, and also prevent photoelectron that bloom from one photodetector from migrating to an adjacent photodetector.
The photodetector is positioned adjacent to the sides and top of each of the pixels and the pixels further include circuitry components positioned below the photodetector, wherein the scavenging diode is positioned along at least one of the sides of the pixels. The scavenging diode can bisect the isolation region between adjacent photodetectors or can be positioned below the isolation region to economize chip area. The scavenging diode is positioned at least between adjacent photodetectors in adjacent pixels. The voltage source and the scavenging diode can comprise a continuous active area region.
The pixels further include circuitry components positioned below the photodetector, wherein the scavenging diode is positioned along the sides, and the circuitry components form an effective scavenging diode below the photodetector.
The invention supplies a scavenging diode which removes photo-generated and thermally generated electrons from the isolation region between adjacent photodetectors. The invention further prevents electrons that are photo-generated within one photodetector from blooming into adjacent photodetectors. Therefore, the adjacent photodetectors primarily collect photoelectrons which actually land upon the photodetectors and do not substantially collect photoelectrons which land adjacent to the photodetectors. Also, with the invention, photo-generated electrons from a first highly illuminated photodetector cannot spill from the first photodetector and migrated to (and be collected by) adjacent photodetectors. This allows for more accurate measurement of the level of photoelectrons received in the area defined by each photodetector, and reduces color channel mixing. This prevention or reduction of color cross-talk improves the color image quality or color fidelity that can be produced by the image sensor.
In addition, the invention inhibits thermally generated electrons in isolation regions surrounding the photodetectors from migrating to and being collected in the photodetectors. This improves the signal to noise ratio of the image sensor.